TA*p63 and GTAp63 achieve tighter transcriptional regulation in quality control by converting an inhibitory element into an additional transactivation domain

Abstract

The p53 homolog p63 plays important roles in development of epithelial tissues and quality control in germ cells. These two functions are executed by two distinct isoforms of p63. They are created by different promotors resulting in isoforms having either an N-terminal transactivation domain (TAp63) or a truncated form (ΔNp63). In addition to these two N-terminal isoforms a third one with an even longer N-terminus, named TA*p63, has been found. A fourth N-terminal isoform, GTAp63, that closely resembles TA*p63 was discovered in male germ cells where it is involved in genetic quality control. Here, we characterize TA*p63α and GTAp63α and show that their N-terminal extensions stabilize the closed and only dimeric conformation adopted by the shorter TAp63α protein. Both proteins can be activated by the two kinases Chk2 and CK1 resulting in the open tetrameric state. In this conformation, the N-terminal extension acts as an additional transactivation domain enhancing transcriptional activity. Through this mechanism, the difference in transcriptional activity between the repressed and the active state of the protein gets enhanced relative to TAp63α. Finally, we show by mass spectrometry that TA*p63α is expressed in the breast cancer cell line Sum159 at the protein level together with mutant p53. Upon doxorubicin treatment, TA*p63α gets activated, providing a potential new tool to fight cancer.

The p63 C-terminus is essential for murine oocyte integrity

“…This isoform is created by the by insertion of the LTR region of the human endogenous retrovirus 9 upstream of the TAp63 gene resulting in the addition of 37 amino acids at the N-terminus. A detailed biochemical analysis of this isoform has shown that this additional sequence results in a further stabilization of the dimeric, inactive state 16 . Why (human) male germ cells require this extra safety mechanism remains an open question but it has been proposed that expression of GTAp63α is important to enable longer reproductive periods in hominids.…”

“…Its activation is essential to eliminate such oocytes suffering DNA-damage-induced death and in this way to maintain the integrity of the female germline; such monitoring plays a crucial role in the quality control of the female ovarian reserve 8 , 13 , 14 . Although TAp63 and yet another, N-terminally elongated isoform, GTAp63α 15 , 16 , has also been detected in male germ cells, its role in this context has not yet been fully clarified 17 – 19 .…”

Isoform-Specific Roles of Mutant p63 in Human Diseases

“…Mice specifically lacking the TAp63 isoform develop metastatic mammary and lung adenocarcinoma [ 147 ], and TAp63α has been found in several breast cancer derived cell lines with mutations in the TA domain [ 148 ]. Recently, another isoform, called TA*p63α, was identified in Sum 159 cells [ 149 ]. This isoform also adopts a closed, only dimeric conformation that, through an N-terminal extension, is even more inhibited than TAp63α [ 14 , 149 ].…”

“…Recently, another isoform, called TA*p63α, was identified in Sum 159 cells [ 149 ]. This isoform also adopts a closed, only dimeric conformation that, through an N-terminal extension, is even more inhibited than TAp63α [ 14 , 149 ]. While clearly more research is required to understand the specific transcriptional programs regulated by different p63 isoforms, it seems likely that, in cells of the basal compartment of mammary glands, the interplay of ΔNp63α and TAp63α is crucial for normal tissue development and its misregulation contributes to the development of breast cancer.…”

DNA Damaged Induced Cell Death in Oocytes

“…At the center of such approaches are the three kinases, ATM, CHK1/2 and CK1, that play essential roles in this activation process. Inhibitor studies in mouse ovaries have indeed shown that using inhibitors for one of these kinases suppresses activation of TAp63α [64,135], allowing oocytes to survive [72,136].…”